1. Field of the Invention
The present invention relates to a light-quantity adjusting apparatus, an optical apparatus and a camera having the light-quantity adjusting apparatus.
2. Description of Related Art
As a conventional light-quantity adjusting apparatus mounted on an image-taking apparatus such as a video camera, a light-quantity adjusting apparatus having the configuration shown in FIG. 18 is known. The light-quantity adjusting apparatus has two blades, and has a configuration in which one rotation type electromagnetic actuator (motor) 23c is used as a drive source to drive two diaphragm blades 23a and 23b through a seesaw-type drive lever 23d. 
An ND (neutral density) filter 23a1 is stuck on the diaphragm blade 23a. When an object is bright, when an aperture (diaphragm aperture) formed by the diaphragm blades 23a and 23b is excessively small in diameter, degradation of image quality caused by diffraction of light and reflection of dust caused by increasing depth of focus are posed as problems. For this reason, the ND filter 23a1 is stuck on the diaphragm blade 23a to position the ND filter 23a1 in the diaphragm aperture of the diaphragm blades 23a and 23b, so that a diaphragm aperture is repressed from being excessively small.
FIGS. 19(A) to 19(H) are diagrams showing continuously movements of the light-quantity adjusting apparatus having the above-described configuration when a diaphragm is driven from an open state to a closed state. In the intermediate states (B) to (F) before a small aperture shown in FIG. 19(G) or 19(H), a pass-through portion where the ND filter 23a1 does not entirely cover the whole area of the diaphragm aperture formed by the diaphragm blades 23a and 23b bring about an effect as if the pass-through portion were a small aperture, and image quality is degraded.
In order to solve this problem, in recent years, an image pickup element decreases in size, and a pixel pitch becomes small to increase the influence of diffraction. For this reason, a light-quantity adjusting apparatus that independently drives two diaphragm blades and ND blades having two or more different concentrations is proposed. FIG. 20 is an exploded perspective view of the light-quantity adjusting apparatus.
In this light-quantity adjusting apparatus, a pivoting arm 101 pivots in response to drive force from a motor 102, two diaphragm blades 103 and 104 move in opposite directions (upward and downward directions in FIG. 20) to change a diaphragm aperture area serving as a light-passing area. When a pivoting arm 105 pivots in response to drive force from a motor 106, an ND filter holding member 108 on which an ND filter 107 is stuck moves in the upward and downward directions in FIG. 20. In this manner, the ND filter 107 moves in the diaphragm aperture formed by the diaphragm blades 103 and 104.
However, in the light-quantity adjusting apparatus described above, since the two motors 102 and 106 are used to drive the diaphragm blades 103 and 104, the light-quantity adjusting apparatus increases in size due to an increase in number of parts.
Therefore, a diaphragm apparatus for driving diaphragm blades and an ND filter by drive force from one drive source while reducing the influence of diffraction is known (for example, Japanese Patent Application Laid-Open No. H11(1999)-64923 (FIGS. 5 and 6)). In this diaphragm apparatus, the influence of diffraction (degradation of image quality) caused by a pass-through portion that becomes minimum in an intermediate aperture obtained before a small aperture is achieved is reduced.
More specifically, the diaphragm apparatus has two diaphragm blades and an ND filter holding member to which a first ND filter is attached, and the ND filter holding member is arranged such that the ND filter holding member is moved in the same direction as that of the other diaphragm blade positioned to be opposed to one diaphragm blade in an optical axis direction. A second ND filter is attached to the other diaphragm blade to partially cover a notch for forming the diaphragm aperture, and an area where the first ND filter covers the diaphragm aperture is made larger than an area where the second ND filter covers the diaphragm aperture, and a displacement amount of the first ND filter depending on a change in aperture diameter is made larger than a displacement amount of the second ND filter.
However, in a light-quantity adjusting apparatus disclosed in Japanese Patent application Laid-Open No. H11-64923, as shown in FIG. 6 of this reference, a section in which a diaphragm aperture change includes a large section in which contrast becomes low. In this case, in the section in which the contrast becomes low, an MTF (Modulation Transfer Function) is deteriorated. If the section in which the contrast becomes low is large, when the light-quantity adjusting apparatus is mounted on an image pickup apparatus, a high-quality image cannot be obtained.